1. Field of the Invention
The present invention relates to a method for forming polymers wherein a micellar polymerization method is utilized. More specifically, the method comprises the incremental addition of a hydrophobic monomer to a reaction medium which includes both a water soluble monomer and a surface active agent, so that hydrophobic monomer is present in a constant concentration throughout the polymerization process.
2. Technology Description
Micellar polymerization is one of several methods that can be used to produce water soluble or water dispersible polymers containing hydrophobic structural features. These polymers, through complex interactions with themselves, colloids, surfaces, interfaces, solvents, electrolytes, and associative structures such as surfactant micelles and vesicles or natural polyelectrolytes with important tertiary structures such as proteins and enzymes are of great technological and economic interest.
Micellar polymerization consists of copolymerizing a mixture of hydrophilic and hydrophobic monomers by a chain growth method wherein the bulk of the monomers are water soluble, a surfactant is used to obtain a thermodynamically stable micellar solution of the hydrophobic monomers, and the resulting product is substantially a water soluble or water dispersible polymer either dissolved in water or dispersed in water.
Micellar solutions are distinguishable from microemulsions in that they can be formed at any surfactant concentration above the critical micelle concentration and do not depend on achieving the very low surface tensions necessary for the formation of a thermodynamically stable microemulsion. The main requirement for the formation of a micellar solution is that the solute be soluble to some extent within the micelle interior, which may include the transition or palisades layer near the surface of the micelle.
A micellar solution is distinguishable from a conventional emulsion in that in the micelle interior there is no essentially homogeneous phase of the solute, the micelle has properties similar to pure surfactant micelles in terms of very small physical size, contain a small number of molecules, often on the order of 100 to 1000, are generally too small to refract light to provide the characteristic white appearance of an emulsion, and possess a very small mean lifetime, on the order of milliseconds. Micellar solutions are microheterogeneous systems that are characteristically isotropic, optically transparent and thermodynamically stable.
Evani (U.S. Pat. No. 4,432,881) and Lacik et al. (Compositional Heterogeneity Effects in Hydrophobically Associating Water-Soluble Polymers Prepared by Micellar Copolymerization, POLYMER, Volume 36, Number 16, 1996, pp. 3197-3211) describe examples of this technique, the resulting products and some of the limitations thereof. This synthesis method is distinct from conventional polymerization processes such as emulsion polymerization where the bulk of the monomers are water insoluble (Hill, Candau, and Selb, Macromolecules, 1993 vol. 26, p 4521; Odian, Principles of Polymerization, 3rd Edition, p335 ff.), or from inverse microsuspension polymerizations (Larson, U.S. Pat. No. 4,617,362) where the continuous phase is immiscible with water, or from polymerizations in microemulsions (Candau, Kinetic Study of the Polymerization of Acrylamide in Inverse Microemulsions, Journal of Polymer Science, Vol. 23, p 193-214, 1985).
Applications of the class of materials produced by micellar polymerization include associative thickeners having the ability to control the rheology of a variety of systems including cosmetics, paints and other aqueous systems, where flow behavior influences the ease and effectiveness of application. Rheology enhancement can also be useful in improving the apparent skin feel or richness of a formulation. Rheology can also be important in enhancing the stability of a colloidal system. Other applications for the products of micellar polymerization include compositions that enhance the foamability of surfactant systems, (see, for example U.S. application Ser. No. 08/573,794) modify the solution behavior of the polymer to provide control of the characteristics of the polymer on exposure to environmental variables (so called `smart polymers`) and provide associative structures with other materials such as physiologically active compounds to control the rates at which they become biologically available. Applications in water treatment by providing a polymer with both flocculant and sorptive properties are possible. Other applications of products possible from the practice of this technology may include development of products that provide tailored surface substantivity, or provide for modification of the surface properties of the materials to which they may become adsorbed to. Polymers of this type have been shown to be particularly effective in solubilizing organic materials in aqueous solutions.
Polymers with surface active properties such as those which can be produced by micellar polymerization are desirable for use in cosmetic formulations as compared to surfactants because of their increased mildness as a result of having higher molecular weights as compared with surfactants. Micellar structures derived from modified polymers can have relaxation times on the order of 10-100 seconds, where surfactant micelles have relaxation times on the order of milliseconds. This can provide kinetically stabilized structures impossible with surfactant micelles. Such structures may be useful for controlling the release of active ingredients, or as microenvironments for carrying out chemical reactions in aqueous media, substituting for less friendly solvent based systems.
Emulsions, particularly complex systems wherein an inner aqueous phase is dispersed in an oil phase which is further dispersed in an external aqueous phase (so-called w/o/w systems) may be stabilized by polymers of this type where surfactant based systems would fail as the polymeric surfactant must desorb many segments from the interface to relocate to migrate between inner and outer surfaces where a surfactant would easily migrate, thereby destabilizing the inner emulsion. It is also possible by modification of a polymer to obtain compatibility with a surfactant system due to elimination of the phase segregation effects that often occur when polymer/surfactant blends are formed. Modified polymeric systems may undergo self-assembly providing utility in applications such as nanolithography and nanofabrication. Hydrophobically modified polymers may form associative structures with biologically active compounds such as enzymes, preserving, enhancing or enabling activity in situations that would normally inactivate the enzyme. Other applications may become apparent to readers familiar with other areas of technological specialization.
Until now the practical use of micellar polymerization has been hampered by several technological and economic problems. The composition drift that occurs during polymerization due to the enhanced reaction rate of the solubilzed monomer in its micellar environment introduces inhomogenaeties into the polymer structure (The Candau reference provides some discussion of this). For many applications of these types of polymers, the comonomer sequence distribution may have a significant effect on the performance of the polymeric material in question. Another major limitation of existing micellar polymerization technology is the problem of the high ratios of surfactant to hydrophobic monomer typically needed to obtain a micellar solution, from 15/1 to 70/1 and to control the length of the hydrophobic monomer sequence. This may prevent use of the polymer both through the cost of the surfactant, and through interference from the surfactant with the intended end use of the polymer. Finally, because of the need to closely match monomer reactivities in order to control composition drift, the practitioner is limited in his choices of monomers, and thus the potential range of technological applicability of this method.
Alternatives to micellar polymerization, such as the use of polymerizable surfactants instead of hydrophobic monomers in micellar solutions can sometimes be used, but these too have their limitations. In general polymerizable surfactants are available in a much more limited range of composition and structure, and are as well generally much more costly than the chemically simpler hydrophobic monomers available in commerce. Therefore the cost and availability problems that limit the range of potential applicability of copolymerizations using this alternative are obvious.
Another route that can be used to produce polymers of this type is by post-reacting either a hydrophobic or hydrophilic species with the opposite type of polymeric species. This requires the polymer and modification group involved have appropriate reactivities, that the polymeric solution is in a physical state conducive to the mixing and heat transfer processes that might be required, that the solvent does not interfere with the intended reaction, and that the resulting distribution of the modifying agent can be made appropriate to the intended application.
Another methodology, that of conducting polymerization in a solvent of properties intermediate to water and the desired hydrophobic material as to provide mutual solubility can be envisioned. Such solvents may be mixtures of water and other components. However such solvents generally have a number of undesirable properties, including high cost, toxicity, flammability, formation of side products with the reactants, chain transfer activity, and a negative environmental impact. In many cases the use of solvents may be restricted by government legislation. It may be necessary to recover these solvents from the product polymer for one or more of these reasons, in which case the additional processing and equipment requirements provide a different scientific and economic barrier to the use of this technology. Solvent recovery from viscous polymer solutions can be a severe engineering problem, and finding processing conditions that do not also lead to polymer degradation may be very difficult or impossible. In many cases it is possible to find a solvent for the desired monomers, but not for both the monomers and product polymer.
Canadian Patent Application 2,003,406 provides an example of the use of aqueous/solvent based systems as a route to hydrophobically modified water soluble polymers. The systems disclosed do not provide a methodology for effective incorporation of higher alkyl methacrylates due to the limited solubility of these monomers in aqueous/alcoholic systems. Polymerization of such higher alkyl monomers in these solvents result in mixtures of a suspension polymer containing the hydrophobic monomers and a water soluble polymer with very minor or insignificant hydrophobic modification. This mixture will not have the valuable performance characteristics of a hydrophobically modified water soluble polymer.
It has long been known (U.S. Pat. No. 2,100,900) that the practice of feeding a more reactive monomer into a reaction system can be used to control the change in composition that often occurs when two monomers are being polymerized. For example, many commonly practiced emulsion polymerizations are accomplished in this manner. However it has not been hitherto appreciated that this technique can be used to overcome some problems associated with the micellar polymerization process, and provide a number of beneficial, and in some cases unobvious results. In particular this method is possible because of the competitive kinetics of transfer to micelles (10.sup.-7 sec) vs. chain growth addition to polymer (10.sup.-4 sec).
Despite the above teachings, there still exists a need in the art for producing a hydrophobically modified water soluble or water dispersible polymer wherein a micellar polymerization technique is utilized which does not suffer from the above described defects.